The equilibrium factor for thoron in the indoor environment

Abstract

It is well known that a half of the annual effective dose from natural radiation sources consists of that from radon. The dose is actually derived not from radon gas itself but from its decay products. In actual surveys, however, radon is subject to measure and the dose can be estimated after obtaining its decay products concentration by multiplying an equilibrium factor. In general, the equilibrium factor of radon is assigned to be 0.4 indoors and 0.6 outdoors as the typical value, respectively. Thoron, formed in Th-232 decay series, is present as well as radon in the environment. Previous nationwide indoor radon surveys and many other studies have shown thoron has been detected in the indoor environment. We have measured indoor radon, thoron, thoron decay products concentrations in various environments. The present study describes a summary of the survey results on the equilibrium factor for thoron.